Photochromic or phototropic glasses, as such have been variously termed, had their genesis in U.S. Pat. No. 3,208,860. That patent generally disclosed the capability of silver halide crystals grown in situ in glass articles to impart photochromic behavior thereto. As stated therein, the preferred base glass compositions consisted essentially, in terms of weight percent on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, and an alkali metal oxide selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of those four components constituting at least 85% of the total composition. The photochromic characteristics exhibited by those compositions resulted from the incorporation of silver and at least one halide selected from the group of Cl, Br, and I. The minimum amount of halide necessary to achieve photochromic properties was 0.2% Cl, 0.1% Br, and 0.08%I. Silver is included in at least the minimum indicated content of 0.2% where Cl is the effective halide, 0.05% where Br is the effective halide, and 0.03% where I is the effective halide. The patent also teaches the efficiency of adding small amounts of low temperature reducing agents, such as SnO, FeO, CuO, As.sub.2 O.sub.3, and Sb.sub.2 O.sub.3 to improve the photochromic behavior of the glass. Reference is made to that patent for a more detailed explanation of the mechanism of photochromism in glasses.
Currently, the largest commercial application for photochromic glass has been the manufacture of spectacle lenses, both as ophthalmic lenses and as non-prescription sunglasses. Prescription lenses, marketed under the trademark PHOTOGRAY by Corning Glass Works, Corning, New York, have comprised the greatest single segment of commercial sales. That glass has the following approximate composition, expressed in terms of weight percent on the oxide basis:
______________________________________ SiO.sub.2 55.6 B.sub.2 O.sub.3 16.4 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.65 Na.sub.2 O 1.85 K.sub.2 O 0.01 BaO 6.7 CaO 0.2 PbO 5.0 ZrO.sub.2 2.2 Ag 0.16 CuO 0.035 Cl 0.24 Br 0.145 F 0.19 ______________________________________
Because the composition of PHOTOGRAY brand lenses resulted from compromises worked out between photochromic behavior and such physical and chemical properties as glass melting and forming capabilities, ophthalmic requirements, chemical durability, weathering resistance, and the like, research has been essentially continuous to produce glasses demonstrating better photochromic characteristics, while concomitantly exhibiting the chemical and physical properties necessary in the formation of ophthalmic products or in products for other applications.
For example, ophthalmic lenses have recently been marketed under the trademark PHOTOGRAY EXTRA by Corning Glass Works. The glass for such lenses darkens to a lower luminuous transmittance and fades to its original transmittance substantially more rapidly than the glass utilized in PHOTOGRAY brand lenses. The composition therefore is included within U.S. Pat. No. 4,190,451, that patent citing glasses consisting essentially, expressed in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 20-65 Al.sub.2 O.sub.3 5-25 B.sub.2 O.sub.3 14-23 Li.sub.2 O 0-2.5 Na.sub.2 O 0-9 K.sub.2 O 0-17 Cs.sub.2 O 0-6 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 8-20 P.sub.2 O.sub.5 0-25 CuO 0.004-0.02 Ag 0.15-0.3 Cl 0.1-0.25 Br 0.1-0.2 ______________________________________
wherein the molar ratio alkali metal oxide:B.sub.2 O.sub.3 ranges between about 0.55-0.85, the composition is essentially free from divalent metal oxides other than CuO, and the weight ratio Ag:(Cl+Br) ranges about 0.65-0.95.
Ophthalmic lenses fabricated from the above compositions have been produced in accordance with the method utilized in the manufacture of conventional lenses, i.e., non-photochromic ophthalmic lenses. Hence, a glass blank is pressed from molten glass and the blank then ground and polished to the required prescription. As pressed, the glass blank is potentially photochromic, i.e., the blank is essentially non-photochromic but, after exposure to an appropriate heat treatment, it will display photochromic behavior when exposed to actinic radiation, principally ultraviolet radiation. Accordingly, at some stage of the lens making process, the glass will be subjected to a predetermined heat treatment to develop the desired photochromic properties.
U.S. Pat. Nos. 4,018,965, 4,130,437, and 4,168,339 disclose glass compositions which can be drawn into glass sheet and which are potentially photochromic in such configuration. Those glass compositions demonstrate a viscosity at the liquidus of at least 10.sup.4 poises, commonly 10.sup.4 -10.sup.6 poises, and manifest long term stability against devitrification when in contact with platinum metal at temperatures corresponding to glass viscosities in the range of 10.sup.4 -10.sup.6 poises. This resistance to crystallization is required to permit the forming of glass sheet since the sheet forming methods conventional in the glass art do not rapidly quench the glass melt as occurs in commercial glass pressing operations. Hence, slow cooling of the glass hazards the development of turbidity or haze in the glass due to the growth of crystals therein.
The operable ranges of glass compositions disclosed in those three patents are tabulated below in terms of weight percent on the oxide basis.
______________________________________ 4,018,965 4,130,437 4,168,339 ______________________________________ SiO.sub.2 54-66 54-66 54-66 Al.sub.2 O.sub.3 7-15 7-15 7-16 B.sub.2 O.sub.3 10-25 10-25 10-30 Li.sub.2 O 0.5-4 0.5-4 0-4 Na.sub.2 O 3.5-15 3.5-15 3-15 K.sub.2 O 0-10 0-10 0-10 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 6-16 -- PbO 0-3 0-1.25 0.4-1.5 Ag 0.1-1 0.1-0.3 &gt;0.03-1 Cl 0.1-1 0.2-1 0.5-1.2 Br 0-3 0-0.3 0.2-0.5 CuO 0.008-0.16 0.002-0.02 0.008-0.03 F 0-2.5 0-2.5 0.2-0.5 ______________________________________
With regard to the photochromic characteristics displayed by the compositions disclosed in each patent, the glasses of U.S. Pat. No. 4,018,965 are asserted to exhibit at ambient temperatures, viz., 20.degree.-25.degree. C., a clear luminous transmittance of at least 60%, a darkened luminous transmittance not exceeding 25%, and a fading rate such that the glass exhibits a faded luminous transmittance after a 5-minute fading interval from the darkened state of at least 1.5 times that of the darkened transmittance. The glasses of U.S. Pat. No. 4,130,437 are reported to display a clear luminous transmittance of at least 60% at temperatures of 20.degree.-25.degree. C., a darkened luminous transmittance below 30%, and a rate of fading such that the glass demonstrates a faded luminous transmittance after a 5-minute fading period from the darkened state of at least 1.75 times that of the darkened transmittance. Moreover, after a one-hour fading interval, the glass manifests a luminous transmittance in excess of 80% of its clear luminous transmittance. The glasses of U.S. Pat. No. 4,168,339 are stated to exhibit a darkened luminous transmittance below 50% at room temperature and a rate of fading such that, after five minutes' fading, the transmittance will have increased by at least 20 percentage units of transmission and, after one hour's fading, the glass will measure a luminous transmittance in excess of 80%.
The initial clear luminous transmittance levels of the glasses are in the vicinity of 90%. Where desired, however, tinting agents such as CoO and/or NiO can be incorporated into the glass compositions to reduce the initial clear luminous transmittance values to levels approaching 60%. Nevertheless, because the amount of tinting agent needed to impart the desired color to the glass is so small, the photochromic behavior of the glass is not significantly affected.
U.S. application Ser. No. 252,139, filed Apr. 8, 1981 in the names of George B. Hares, David J, Kerko, and David L. Morse under the title Photochromic Glass Suitable for Microsheet and Simultaneous Heat Treatment and Shaping, discloses glasses having a clear luminous transmittance of about 90% when free from tint, a darkened luminous transmittance below 25% at a temperature of 20.degree. C., when free from tint and below 20% when the glass is tinted to a level of at least 75%, a fading rate at 20.degree. C. such that the glass displays a faded luminous transmittance of at least twice that of the darkened transmittance after a five-minute fading interval from the darkened state, a darkened luminous transmittance at 40.degree. C. below 45% when the glass is free from tint and below 40% when the glass is tinted to a level of at least 75%, and a fading rate at 40.degree. C. such that the glass manifests a faded luminous transmittance of at least 1.75 times that of the darkened transmittance after a five-minute fading interval from the darkened state. Those glasses consist essentially, in terms of weight percent on the oxide basis,
______________________________________ SiO.sub.2 55-60 Al.sub.2 O.sub.3 9-10 B.sub.2 O.sub.3 19-20.5 Li.sub.2 O 2-2.5 Na.sub.2 O 2-3 K.sub.2 O 6-7 PbO 0.1-0.25 Ag 0.1-0.15 Cl 0.3-0.5 Br 0.05-0.15 CuO 0.0065-0.01 ______________________________________
U.S. Pat. No. 4,102,693 discloses photochromic glasses having a half fading time of not more than one minute which have compositions free from barium and consist essentially, in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 31-59 B.sub.2 O.sub.3 18-28 Al.sub.2 O.sub.3 8-20 Li.sub.2 O 0-3 Na.sub.2 O 0-8 K.sub.2 O 0-16 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 MgO 0-2.6 P.sub.2 O.sub.5 0-12 ZrO.sub.2 0-7 TiO.sub.2 0-5 PbO 0-7 Ag.sub.2 O 0.05-0.4 Cl 0.04-0.5 Br 0-1.0 F 0-0.2 Cl + Br + F 0.13-1 CuO 0-1 ______________________________________